Veritas File System 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

VxFS Performance: Creating, Mounting, and Tuning File Systems
I/O Tuning
Chapter 252
For an application to do efficient disk I/O, it should issue read requests that are equal to the product of
read_nstream multiplied by read_pref_io. Generally, any multiple or factor of read_nstream
multiplied by read_pref_io should be a good size for performance. For writing, the same rule of thumb
applies to the write_pref_io and write_nstream parameters. When tuning a file system, the best
thing to do is try out the tuning parameters under a real life workload.
If an application is doing sequential I/O to large files, it should try to issue requests larger than the
discovered_direct_iosz. This causes the I/O requests to be performed as discovered direct I/O
requests, which are unbuffered like direct I/O but do not require synchronous inode updates when extending
the file. If the file is larger than can fit in the cache, using unbuffered I/O avoids removing useful data out of
the cache and lessens CPU overhead.